This invention relates to analog-to-digital converters used in industrial process control transmitters, and particularly to mapping an analog input range of a delta-sigma converter to the output range of an analog sensor.
Various analog-to-digital (A/D) converters are known that convert the analog output of a sensor to a digital output. A/D converters generate a combination of duty cycle, timing, frequency or pulse counts that digitally represent the output of the sensor. One known type of A/D converter is a delta-sigma circuit, also known as sigma-delta, xcex94xcexa3, and xcexa3xcex94 circuits. Delta-sigma converters are balanced A/D converter circuits that are distinguished from other balanced A/D circuits by the provision of a balancing current to an integrator. The polarity of the balancing current is controlled by a clocked controller. The delta-sigma circuit is capable of producing high accuracy, high resolution analog-to-digital conversions over its analog input range. Delta-sigma circuits are often employed in industrial process control transmitters to convert an analog signal from the sensor to a digital signal so that information concerning the process variable can be transmitted to a central station remotely located from the transmitter.
In practice, however, a sensor, such as a strain gauge, will often have an output range that has a low level and is offset relative to the range of the converter. For example, a strain gauge. operates to provide an output having a range between 0 volts and some maximum sensor voltage (+Vmax), whereas the delta-sigma converter provides an output that ranges between a negative maximum voltage and a positive maximum voltage. When the sensor is directly coupled to the converter, there is a mismatch, and the half of the high resolution capability of the delta-sigma converter is lost for the sensor application.
To overcome this. loss of resolution, it is common to employ analog conditioning circuitry, such as amplifiers and offset circuits, between the sensor output and the input of the delta-sigma converter. The conditioning circuitry, however, together with noise and temperature drift associated with the measurement, adversely affect the high accuracy of the delta-sigma converter for the sensor application. Moreover, only a limited amount of power is available to industrial process control transmitters, so the power consumed by the conditioning circuitry diminishes the availability of power to the transmitter for other purposes. Accordingly, a method and circuit are needed that allow an analog sensor with a low level or offset analog output to be coupled directly to a delta-sigma converter having a higher level input range, to utilize substantially the full range of the delta-sigma converter without the inaccuracies of analog amplifying and offsetting circuits.
A delta-sigma converter includes a mapping circuit coupled to the integrator circuit to map the analog input range of the integrator to the analog output range of the sensor.
More particularly, an integrator circuit has an analog input range and provides an integrator output to a controller to generate a digital output. The digital output is in a digital output range representing the analog input range. A sensor input circuit includes a sensor having an analog sensor output range. The mapping circuit is coupled to the integrator circuit and is responsive to control signals from the controller to map the analog input range to the analog sensor range.
In preferred embodiments, the integrator circuit has at least two differentially arranged inputs. The sensor input circuit includes a pair of charge packet generators each coupled to a sensor capacitor to supply charges of opposite polarity to the integrator circuit inputs. The mapping input circuit includes a pair of charge packet generators each having a mapping capacitor and arranged to alter the charge supplied by the sensor first charge packet generators to adjust the scale of the integrator analog input range to the analog output range of the sensors.
In other embodiments, a reference circuit provides a reference charge for the integrator inputs.